Much of the seismic activity of northern Sweden consists of micro-earthquakes occurring near postglacial faults. However, larger magnitude earthquakes do occur in Sweden, and earthquake statistics indicate that a magnitude 5 event is likely to occur once every century. This paper presents dynamic analyses of the effects of larger earthquakes on an upstream tailings dam at the Aitik copper mine in northern Sweden. The analyses were performed to evaluate the potential for liquefaction and to assess stability of the dam under two specific earthquakes: a commonly occurring magnitude 3.6 event and a more extreme earthquake of magnitude 5.8. The dynamic analyses were carried out with the finite element program PLAXIS using a recently implemented constitutive model called UBCSAND. The results indicate that the magnitude 5.8 earthquake would likely induce liquefaction in a limited zone located below the ground surface near the embankment dikes. It is interpreted that stability of the dam may not be affected due to the limited extent of the liquefied zone. Both types of earthquakes are predicted to induce tolerable magnitudes of displacements. The results of the postseismic slope stability analysis, performed for a state after a seismic event, suggest that the dam is stable during both the earthquakes

Numerical analyses are presented in this thesis to address potential stability problems that may occur during gradual raisings and under seismic loading conditions of Aitik tailings dam in northern Sweden. The dam is mainly raised using upstream construction method. It is planned to raise the dam gradually in several stages. Two dam parts were studied. The first dam part is a straight dam portion, and the second dam part is a corner. The main concerns associated with future raisings of the straight dam part were: (i) the stability of the dam could be affected by an increase in excess pore pressures during sequential raisings, (ii) how to gradually strengthen the dam by using rockfill berms as supports in such a way that required slope stability can be achieved with a minimum volume of rockfill berms, and (iii) if the dam is subjected to seismic loading, whether or not an increase in excess pore pressures could lead to extensive liquefaction which may cause a failure. The problems related to the dam corner were that tension zones and/or low compression zones could develop because of the horizontal pressure of the stored tailings on the inside of the curvature of the dam corner.Numerical analyses were conducted on both the dam parts using finite element method. Two dimensional (2D) plane strain finite element model was utilized to analyse the straight dam. The dam corner was analysed with both the three dimensional (3D) finite element model, and the 2D axisymmetric finite element model. Coupled deformation and consolidation analyses, and slope stability analyses were performed on both the dam parts to simulate gradual raisings, and to compute safety factors. In addition to this, dynamic analyses were carried out on the straight dam part to evaluate the potential for liquefaction, and seismic stability of the dam. The seismic behaviour of the dam was analysed for two cases: (i) a normal case (earthquake of 3.6 Swedish local magnitude), and (ii) an extreme case (earthquake of 5.8 moment magnitude).The results of the straight dam part, with only previously existing rockfill berms, indicate that stability of the dam was reduced due to an increase in excess pore pressures during raisings. Rockfill berms were utilized as supports to raise the dam with enough safety. An optimization technique was utilized to minimize the volume of rockfill berms. This technique could result in significant saving of cost of rockfill berms.The results of the dam corner show that tension zones and/or low compression zones were located on the surface of the dam corner, mainly above the phreatic level. It is interpreted that there is no risk of internal erosion through the embankments because no seepage path occurs above the phreatic level, and a filter zone exists along the slope of the dam. It is suggested to gradually strengthen the dam corner with rockfill berms. The results of the 2D axisymmetric analyses of the dam corner were in a fairly good agreement with those of the 3D analyses. This implies that the 2D axisymmetric analyses are valid for this dam corner. This is an important finding as 2D axisymmetric analyses require much less computational time compared to 3D analyses.The results of the dynamic analyses performed on the straight dam (including additional rockfill berms) suggest that, for the extreme case, liquefaction could occur in a limited zone that is located below the surface near the embankments. For both the normal and the extreme case, (i) seismically induced displacements seem to be tolerable, and (ii) the post seismic stability of the dam is considered to be sufficient.The findings of this study have been practically applied to the Aitik tailings dam. In general, the modelling procedure and the optimization technique to minimize volume of rockfill berms, presented in this study, could be applied to other tailings dams.

The consolidation process could be slow in an upstream tailings dam; therefore, the stability can reduce due to an increase in excess pore pressures when the dam is raised. The safety of the dam can be enhanced by constructing rockfill berms on the downstream side. This paper presents a case study on the strengthening of an upstream tailings dam with rockfill berms. The finite element analyses were performed for modelling the staged construction of the dam and for optimizing the volume of the rockfill berms. The dam was raised in 11 stages; each stage consisting of a raising phase and a consolidation phase. The study shows that the slope stability of the dam reduced due to an increase of excess pore pressures during the raising phase. The stability of the dam was successfully improved by utilizing rockfill berms as supports on the downstream side. A technique has been presented to minimize the volume of the rockfill berms so that the required stability can be achieved at minimum cost. This paper shows that the finite element method can be a useful tool for modelling the consolidation behaviour of an upstream tailings dam and minimizing the volume of the rockfill berms that may be needed to maintain the stability of the dam during staged construction.

A tailings dam is a large embankment structure that is constructed to store the waste from the mining industry. Stability problems may occur in a tailings dam due to factors such as quick rate of raising, internal erosion and liquefaction. The failure of a tailings dam may cause loss of human life and environmental degradation. Tailings Dams must not only be stable during the time the tailings storage facility is in operation, but also long time after the mine is closed. In Sweden, the licensing authorities demand that the tailings dams should be stable up to the next glaciation age, which is interpreted to be 1000 years or more. This goal requires extensive knowledge in order to be able to design the tailings dams so that they can be stable for such a long time. At the Division of Mining and Geotechnical Engineering at Luleå University of Technology, research has been conducted on the stability of tailings dams, using the finite element method which is considered to be suitable for modelling the complex geometry and the material properties of tailings dams. A case study is presented which describes the application of the finite element method in evaluating the stability of a tailings dam.

The 3D finite element analyses were performed for the gradual raisings of a corner section of Aitik tailings dam, located in the north of Sweden. The purpose of the analyses was to investigate the potential risk of low compressive stresses or tensile stresses in the corner and to evaluate the slope stability of the dam for current and future raisings. The results indicate that the magnitudes of the minor effective principal stresses in the interior of the dam are sufficient to resist the development of soft zones or cracks in the dam and it is interpreted that there is no increased risk of internal erosion. The slope stability analyses show that the dam has enough safety up to a height of 76 m, if the dam is gradually strengthened with rockfill supports on the downstream side.

A curved embankment (corner) of an upstream tailings dam was analyzed with the finite element method to identify possible zones of low compressive stresses susceptible to hydraulic fracturing that might initiate internal erosion. The embankment was also analyzed as a straight section, with the same cross section as in the corner, in order to compare compressive stresses in the corner and the straight section. The analysis showed that in comparison to the straight section of the dam, the compressive stresses in the corner were (i) much lower above the phreatic level, in the rockfill banks and the filter zones, and (ii) fairly lower below the phreatic level. The rockfill and the filter contain coarse materials, which are not sensitive to hydraulic fracturing and internal erosion. An increase in radius of the corner is proposed to avoid too low compressive stresses that may develop due to future raisings. The slope stability analysis showed that the corner is currently stable, but an additional rock fill bank on the downstream toe is required for future raisings.

Upstream tailings dams may experience slope stability problems when the rate of raising is too fast. Tailings consolidate slowly due to low hydraulic conductivity. The excess pore pressures can build up due to accelerated rate of raising. The cumulative increase in excess pore pressures due to successive raisings can endanger slope stability of a tailings dam. The stability of a tailings dam is closely related to the consolidation process. The consolidation process and associated stability of an upstream tailings dam during staged construction was modelled with the finite element program PLAXIS. The analysis indicated that the stability of the dam reduced during raising due to increase of excess pore pressures. The safety of the dam was improved by adding rockfill banks on the downstream side. The volume of the rockfill banks was minimized with an optimization technique. This technique involves (i) construction of a rockfill bank on the downstream side when the factor of safety is less than a permissible limit, (ii) utilization of a minimum volume of the rockfill that is necessary to stabilize the slope. This technique can be practicable when the rate of raising is moderate, and partial consolidation occurs between consecutive raisings. Numerical analysis was also performed on a curved embankment of an upstream tailings dam in order to investigate the possible risk of hydraulic fracturing and internal erosion in a corner of the dam. The analysis showed that low compressive stresses occurred above the phreatic level, near the zones of filter and rockfill banks. These zones contain coarse material, and are, therefore, not susceptible to hydraulic fracturing and internal erosion. An increase in the radius of the corner is suggested in order to prevent large reductions in compressive stresses that may occur due to future raisings. Presently the curved dam section is stable. However, an additional rockfill bank on the downstream side will be required for future raisings. Static liquefaction is considered as a common cause of disastrous flow failures of tailings dams. These flow failures can be predicted with numerical modelling using suitable constitutive models. In this context, some constitutive models capable of simulating static liquefaction behaviour of loose saturated sands are reviewed. It is generally concluded that the finite element method can be a helpful tool for modelling stability of tailings dams.

Tailings are the waste products produced during extraction of minerals from the ore. The particle size of tailings varies from medium sand to silt or clay size. Tailings are used as a construction material for raising of embankments called tailings dams. During spigotting, the coarse particles (sands) lie close to the embankment, while the fine particles (slimes) move downwards to the impoundment. The rate of failure in tailings dams is higher than that for the conventional water dams. In the past, tailings dams have failed due to various causes such as meteorological incidents, slope instability, piping/seepage, weak foundations, and seismic liquefaction etc. The statistics shows that the tailings dams are not safe even in their construction stages. The closure and reclamation measures require that the tailings dams and impoundments should be stable for a long time (more than 1000 years). Hence, the proper understanding of the mechanical properties (i.e., permeability, stiffness and strength) of tailings is essential in order to make reliable predictions for the long time stability of tailings dams and impoundments. The purpose of the literature survey carried out is: (a) to understand the mechanical properties of tailings material regarding anisotropy, cyclic loading, particle crushing and creep effects; (b) to know the current design practice and identify the factors that may have strong influence on the long time stability of tailings dams and impoundments, and (c) to find from literature, an appropriate constitutive model that can predict realistic behaviour of tailings material. There is similarity in properties of coarse tailings and loose to medium dense natural sands. Slimes are complex material and may show resemblance to natural sands, clays, or a combination of both. The hydraulic conductivity of tailings varies from point to point in a deposit. Tailings are more compressible than the natural soils. The coefficient of consolidation of slimes is in the range shown by natural clays. Due to high particle angularity, the sands and slimes show higher drained shear strength than that for similar natural soils. Undrained strength of sands is important in evaluation of liquefaction behaviour. The embankments can be raised with upstream, downstream or centreline construction methods. The upstream method is susceptible to liquefaction, whereas the downstream and centreline methods are relatively seismic resistant. The stability of the tailings embankments can be enhanced by keeping the phreatic surface low within the embankment body. The phreatic surface can be lowered with the application of cores, drainage zones, and the use of sand tailings in the embankment construction. Cut-off trenches, slurry walls, grout curtains, and liners can control seepage in tailings dams.The pore water pressures develop during construction of tailings embankments. These pore pressures are described as initial static pore pressure, initial excess pore pressure, and pore pressure due to shearing. The stability analyses conditions for conventional water dams (such as end of construction, staged construction, and long term) are also applicable to the tailings dams. The end of construction condition, and staged construction condition of a tailings dam can be analysed with undrained strength analysis. The drained analysis can be used for the long-term condition of a tailings dam (when an embankment attains maximum height and is constructed slowly). The aim of reclamation measures for tailings dams and impoundments is to achieve long time mass stability, environmental safety, and productive land use. Tailings impoundments can be stabilized with ripraps, chemicals, vegetation, and dry/wet covers etc.Most natural soils show anisotropy in strength due to their depositional history. Due to low density and high degree of saturation, tailings show large cyclic strains in a few cycles of stress reversal. The cyclic strength of tailings can increase with decreasing void ratio. Creep occurs in all soils. Clays show more creep than sands. Loose sands creep more than dense sands. The loads due to high tailings embankments may cause particle crushing and creep. The long time stability of tailings dams and impoundments can be predicted with finite element analysis with suitable constitutive models. The numerical analysis needs to be carried out for severe conditions of rainfalls, floods, and earthquakes.